The Synchronous Digital Hierarchy ("SDH") protocol provides a set of signals and procedures defined by ITU-T for governing and supporting telecommunications network management ("TMN") functions. The types of transmission signals and frame structures implemented in SDH is documented in ITU-T Recommendation G.707, the contents and disclosure of which is incorporated by reference as if fully set forth herein. (In the USA th e same set of signals is called "SONET"). Pursuant to ITU-T, one broad TMN function is Performance Management which, inter alia, provides for performance monitoring of telecommunications network components.
Generally, SDH signals comprise a nested set of frame structures with an outer frame structure including the Synchronous Transport Module ("STM-n") frame structure and within the STM-n frame, higher order virtual container "VC-n" frame structures can be present. A VC frame consists of a block of overhead and a payload block. The overhead is added to monitor the transmission performance through the network. Additionally, within a higher order VC frame, lower order VC frame structures can be present. For instance, according to the current SDH standards, there are five VC frame structures: VC-11, VC-12, VC-2, VC-3, VC-4 with VC-3 and VC-4 comprising the higher order VCs and VC-11, VC-12, VC-2 and VC-3 comprising the lower order VCs. It should be understood that a normal VC signal is a VC that contains a client signal in its payload. The VC overhead block contains so called "path trace bytes" which are used to transport an Access Point Identifier (API), which is unique in the network and identifies the source of the path.
In accordance with the ITU-T standards, an unequipped VC ("U-VC-n") is defined as a VC signal of which the path signal label, the path trace identifier and the network operator bytes/bits are all O's, the bit interleaved parity is valid, and the remainder of the VC signal is unspecified and indicates that there is no end-to-end path through the network. The path trace byte is thus set to all-0's in an unequipped VC signal. Typically, the unequipped VC signal is inserted by a switch fabric in the absence of a matrix connection (i.e., input signal). It is recognized by the presence of all-0's in the signal label (in the VC overhead). A supervisory-unequipped VC ("SU-VC-n") is a test signal containing the VC overhead and an undefined payload. It is used to test a connection which is allocated, e.g., to indicate that other applications may exist for this SU-VC-n signal, as a standby connection in the case that one of the active connections in the network fails, and the transport service has to be quickly restored. It is understood that the normal VC and supervisory-unequipped VC signals have path trace bytes carrying the APIs (not all-0's), however, the signal label of the supervisory-unequipped VC signal is also defined as containing the all-0's pattern, i.e. it contains the same signal label pattern as the unequipped VC signal label pattern. This provides a problem for the monitoring equipment as will be explained.
As shown in FIG. 1, a signal path 10 in a telecommunications system comprises a source terminal equipment 20a and a sink terminal equipment 20b and intermediate equipment indicated as elements 25a, 25b, . . . , 25n. A VC-n signal originates at the source terminal equipment 20a and ends at sink equipment 20b where the VC overhead of the passed through VC-n signal is removed and the payload signal is extracted and forwarded. Non-intrusive monitors (NIMs), such as NIM 100 shown in FIG. 1, are provided for monitoring the quality of the VC-n signals as they are transported throughout the network path. NIMs perform a non-intrusive "listen-only" function and can be located at any point in the network path, e.g., in intermediate equipment, through which the VC-n signals pass. Presently, the NIM is able to monitor normal VC-4, VC-3, VC-2, VC-12 and VC-11 signals at any intermediate point in a path. The specifications for all equipment handling SDH VC-n signals are governed by ITU-T Recommendation G.783 and ETS 300 417, the contents and disclosure of which is incorporated by reference as if fully set forth herein.
As NIMs function similarly to path terminations, they behave similarly to the reception of supervisory-unequipped VC signals as VC-n path terminations do. Thus, if a supervisory-unequipped VC-n signal is received by a NIM that conforms with standard, then the existing NIM detects what is commonly referred to as an UNEQ defect ("dUNEQ") which is the detection of a VC signal loss condition. A receipt of a dUNEQ signal means that somewhere in the path between the termination source and sink points, a switch fabric removed the matrix connection, resulting in the switch fabric not being able to pass the incoming VC signal causing the generation of the unequipped VC signal output (See e.g. ETS 300-417-4-1, S4m.sub.-- TT.sub.-- Sk function, or G.783 Snm.sub.-- TT.sub.-- Sk function). NIMs were defined in this way so that a VC-n test signal would not require a change in the existing path termination and non-intrusive monitor functions, in the absence of an application validating such change.
FIG. 2 illustrates a block diagram of a VC-n Layer NIM monitor 100 implementing, e.g., an Snm.sub.-- TT.sub.-- Sk function. As shown in FIG. 2, the existing (normal) VC-NIM 100 includes a dUNEQ detection block 30 which generates a defect signal called "dUNEQ" that is input to the normal "defect correlation" process block 40 and "consequent action" process block 50 at the detection of a signal loss condition, i.e., when the signal label of the received VC-n signal contains the all-0's signal pattern.
The defect correlation process block 40 has multiple input signals amongst others the dUNEQ signal. The process determines, for the case multiple input signals are active, the highest priority defect condition, and output that as the (most probable) "fault cause."
The consequent action process block 50 has multiple input signals, amongst others the dUNEQ signal. The process determines which consequent action should be initiated on activation of an input signal. In general, consequent actions are:
the insertion of the Alarm Indication Signal (AIS) in the downstream direction of transmission to prevent that downstream of this point alarms are being raised; PA1 the insertion of a remote defect indication (RDI) signal in the upstream direction to inform die source of the failed signal that the signal failed; PA1 the insertion of a remote defect indication (RDI) signal in the upstream direction to inform the source of the failed signal that that signal failed; PA1 the generation of a signal fail (SF) indication to indicate, e.g., protection switching restoring the end-to-end transport; PA1 the generation of a signal degrade (SD) indication to initiate, e.g., protection switching restoring the end-to-end transport, etc.
A path termination function is required to support al the mentioned consequent actions. A non-intrusive monitor is not allowed to insert AIS, RDI.
The Trace Id acceptance block 35 is provided to detect the path trace bytes of the received VC-n signal and generate an AcTI signal used later for forwarding of the VC-n signal. The Trade Id acceptance block 35 extracts the path trace bytes from the received VC signal and performs a persistency check on those. If the trace identifier pattern which is extracted is persistent, this pattern becomes the Accepted Trace Identifier (AcTI) signal. The AcTI signal is reported on request of a management system. The management system or the user of this system can then use this information as input for a fault localization action, or as input for a path setup action.
Currently, it is the case that an NIM for a normal VC-n signal detects a signal loss condition upon receipt of a supervisory-unequipped SU-VC signal as well as unequipped VC-n signal due to the all-0's signal label pattern. To rectify this situation, the Q.9/15 standard committee proposed the addition of a dedicated supervisory-unequipped VC-n NIM (SU-NIM) to the G.783 standard as an additional way of monitoring non-intrusively supervisory-unequipped VC signals. This is a consequence of the extension of the application for the supervisory-unequipped VC signal. Thus, a dedicated supervisory-unequipped VC NIM does not detect a signal loss condition when it receives a supervisory-unequipped VC signal.
The definition of the SU-NIM implies that operators would require both NIM as well as SU-NIM to be present in equipments. The operator must now provision a priori which of the two NIM types to activate; either the (normal) NIM or the SU-NIM.
If the operator selected the NIM and a SU-VC-n is actually received (by mistake), a signal loss condition is reported. If instead, the SU-NIM was activated, a connection mismatch condition would have been reported. If the operator would have selected a SU-NIM (instead of a normal NIM), VC-n signal loss condition reporting depends on the provisioning status of connection mismatch condition detection (this is an option controlled by the operator). With connection mismatch, condition detection disabled, the SU-NIM will not be able to detect the signal loss condition, whereas the normal NIM will detect it.
The type of non-intrusive monitor which must be active has to be defined in conjunction with the expected VC signal (normal-VC or test-VC). It must be changed if the expected signal is changed.
The complication for the equipment design is, thus, that two types of NIM processing are to be included, and the actual active NIM type is selectable by the operator.
The complication for the operator using the equipment is, thus, that he/she has to select one of the two NIM types to be active, and that a reselection is required if the VC signal type changes.
The invention merges the behavior of both NIM types, such that a manufacturer needs to implement a single type only, and that an operator doesn't have to select and/or reselect the NIM type depending on the VC signal type which is expected to pass through.
It would thus be desirable to eliminate the need to reprovision the NIM type (normal NIM, SU-NIM) when the normal VC-n signal on the connection is replaced by the supervisory-unequipped VC-n signal.